The Heart’s External Anatomy & Conduction System

Atria (R & L) contract simultaneously Heart at rest Blood flows from large veins into atria Passive flow from atria into ventricles Atria (R & L) contract simultaneously Blood forced into ventricles Ventricles (R & L) contract simultaneously Atrioventricular valves close  “lubb” sound Blood forced into large arteries Ventricles relax Semilunar valves close  “dub” sound

Pericardium Membrane sac Surrounds the heart Protection Anchors Contains serous fluid HEART Pericarditis inflammation of the pericardium decreases serous fluid causing painful adhesions interfering with heart movements A 2 layered bag the outer layer is called the fibrous pericardium and the serous pericardium The fibrous pericardium anchors the heart to surrounding structures like the diaphragm and the sternum. The serous pericardium secrets a fluid contained within the pericardium allows the heart to beat easily in a relatively frictionless environment Pericarditis is when the pericardium becomes inflamed causing the layers of the sac to stick to each other and interfere with heart movemnts Pericardium

Heart Wall Epicardium (outside) – visceral layer of the serous pericardium. Myocardium (muscle) – cardiac muscle layer forming the bulk of the heart. Endocardium (within) – endothelial layer of the inner myocardial surface. Remember “visera” means internal part of a structure or the organs The bulk of the walls of the heart is the myocardium which consists of thick bundles of cardiac muscle twisted and whorled into ringlike arrangements. This is the layer that actually contracts The endocardium is a thin membrane that lines the heart chambers. Endocarditis is inflammation of this lining – what sorts of problem could this cause?

Cardiac Muscle Specialized muscle cells Involuntary Striated Cushioned by endomysium Joined by intercalated discs Cardiac muscle cells are branching cells that are joined by special junctions called intercalated discs, this along with the spiral arrangement of the muscle bundles allow heart activity to be closely coordinated. Why is it important the muscles of the heart be coordinated? The cardiac muscle cells are set up to be fatique resistant by having large mitochondria and having the ability to burn both sugar and fat for energy Cardiac cell metabolism Areobic Large mitochondria Organic fuels: fatty acids & glucose Fatigue resistance

Coronary Arteries Branch off aorta above aortic semilunar valve Left coronary artery supplies left atrium and left ventricle Anterior interventricular artery supplies both ventricles Right coronary artery supplies right ventricle Posterior interventricular artery

Coronary Veins Collects wastes from cardiac muscle Drains into a large sinus on posterior surface of heart called the coronary sinus Coronary sinus empties into right atrium

The heart beats because of the spread of electrical impulses to the heart muscle, causing it to contract.

Cardiac Conduction System Cardiac muscle tissue exhibits autorhythmicity = generates its own stimulation. This is possible because of an internal cardiac conduction system which can initiate and distribute electrical impulses.

Cardiac Conduction System Comprised of interconnected structures Sinoatrial node Atrioventricular node Atrioventricular Bundle Bundle Branches Purkinje Fibres

Sinoatrial (SA) Node Natural Pacemaker Upper RA Neuromyocardial cells Sympathetic & parasympathetic Sympathetic ↑HR Parasympathetic ↓HR

Atrioventricular (AV) Node Junction of atria and ventricles Spread of depolarisation - from atrial myocardium Delay 0.15 seconds Time atria to expel blood Time for ventricular filling Protection to ventricles Less autonomic nervous control than SA node Sympathetic ↑conduction time Parasympathetic ↓conduction time Atrioventricular node

Linked to the nervous system The cardiovascular center of the medulla oblongata connects to the SA & AV nodes directly via the vagus nerve (cranial nerve 10) as well as thru the spinal cord. Sensory information is sent to the cardiac center of the medulla which can illicit a parasympathetic response directly or a sympathetic response thru the spinal cord/ Nervous control from the cardiovascular center in the medulla Sympathetic impulses increase heart rate and force of contraction parasympathetic impulses decrease heart rate. Baroreceptors (pressure receptors) detect change in BP and send info to the cardiovascular center

Depolorization Depolarization begins in sinoatrial (SA) node The heart is autorhythmic Depolarization begins in sinoatrial (SA) node Spread through atrial myocardium Results in myocardial contration of the atria Delay in atrioventricular (AV) node To the Bundle of His AKA atrioventricular bundle The heart is autorythmic – it can carry impulses

Depolorization Entire musculature depolarizes quickly The heart is autorhythmic Separates into 2 main branches left & right Located in the interventricular septum Left bundle – antero-superior division Right bundle – postero-inferior division Bundle branches divide - small, dense network of conduction tissue called the Purkinje Fibers Entire musculature depolarizes quickly

Electrocardiogram Variations in electrical potential radiate from the heart ECG records electrical events in the heart.

P wave QRS complex T wave P-Q interval Q-T interval P-P = one cardiac cycle P-Q = time for atrial depolarization Q-T = time for ventricular depolarization T-P = time for relaxation P wave Depolarization of atria Followed by contraction QRS complex 3 waves (Q, R, & S) Depolarization of ventricles T wave Repolarization of ventricles P-Q interval Time atria depolarize & remain depolarized Q-T interval Time ventricles depolarize & remain depolarized

SA node Represented on the ECG as P wave QRS SA node Represented on the ECG as P wave AV node conduction is represented on the ECG as the PR Interval The Bundle Branch and purkinje fibre depolarisation constitutes ventricular depolarisation Represented on the ECG as the QRS Atrial repolarisation occurs within the QRS & therefore is masked Ventricular repolarisation is represented on the ECG as a T wave

1) atrial depolarization begins 2) atrial depolarization complete (atria contracted) 3) ventricles begin to depolarize at apex; atria repolarize (atria relaxed) 4) ventricular depolarization complete (ventricles contracted) 5) ventricles begin to repolarize at apex 6) ventricular repolarization complete (ventricles relaxed)